1. Field of the Invention
This invention relates to an image forming apparatus such as an image display apparatus using an electron source.
2. Description of the Related Art
Two types of elements, namely hot cathode elements and cold cathode elements, are known as electron emission elements for constructing the electron sources mentioned above. Examples of cold cathode elements are surface-conduction electron emission elements, electron emission elements of the field emission type (abbreviated to "FE" below) and metal/insulator/metal type (abbreviated to "MIM" below).
An example of the surface-conduction electron emission element is described by M. I. Elinson, Radio. Eng. Electron Phys., 10, 1290, (1965). There other examples as well, as will be described later.
The surface-conduction electron emission element makes use of a phenomenon in which an electron emission is produced in a small-area thin film, which has been formed on a substrate, by passing a current parallel to the film surface. Various examples of this surface-conduction electron emission element have been reported. One relies upon a thin film of SnO.sub.2 according to Elinson, mentioned above. Other examples use a thin film of Au [G. Dittmer: "Thin Solid Films", 9, 317 (1972)]; a thin film of In.sub.2 O.sub.3 /SnO.sub.2 (M. Hartwell and C. G. Fonstad: "IEEE Trans. E.D. Conf.", 519 (1975); and a thin film of carbon (Hisashi Araki, et al: "Vacuum", Vol. 26, No. 1, p. 22 (1983).
FIG. 17 is a plan view of the element according to M. Hartwell, et al., described above. This element construction is typical of these surface-conduction electron emission elements. As shown in FIG. 17, numeral 3001 denotes a substrate. Numeral 3004 denotes an electrically conductive thin film comprising a metal oxide formed by sputtering and is formed into a flat shape resembling the letter "H" in the manner illustrated. The conductive film 3004 is subjected to an electrification process referred to as "electrification forming", described below, whereby an electron emission portion 3005 is formed. The spacing L in FIG. 17 is set to 0.5.about.1 mm, and the spacing W is set to 0.1 mm. For the sake of illustrative convenience, the electron emission portion 3005 is shown to have a rectangular shape at the center of the conductive film 3004. However, this is merely a schematic view and the actual position and shape of the electron emission portion are not necessarily represented faithfully here.
In above-mentioned conventional surface-conduction electron emission elements, especially the element according to Hartwell, et al., generally the electron emission portion 3005 is formed on the conductive thin film 3004 by the so-called "electrification forming" process before electron emission is performed. According to the forming process, a constant DC voltage or a DC voltage which rises at a very slow rate on the order of 1 V/min is impressed across the conductive thin film 3004 to pass a current through the film, thereby locally destroying, deforming or changing the property of the conductive thin film 3004 and forming the electron emission portion 3005, the electrical resistance of which is very high. A crack is produced in part of the conductive thin film 3004 that has been locally destroyed, deformed or changed in property. Electrons are emitted from the vicinity of the crack if a suitable voltage is applied to the conductive thin film 3004 after electrification forming.
Known examples of the FE type are described in W. P. Dyke and W. W. Dolan, "Field emission", Advance in Electron Physics, 8,89 (1956), and in C. A. Spindt, "Physical properties of thin-film field emission cathodes with molybdenum cones", J. Appl. Phys., 47, 5248 (1976).
A typical example of the construction of an FE-type element is shown in FIG. 18, which is a sectional view of the element according to Spindt, et al., described above. The element includes a substrate 3010, emitter wiring 3011 comprising an electrically conductive material, an emitter cone 3012, an insulating layer 3013 and a gate electrode 3014. The element is caused to produce a field emission from the tip of the emitter cone 3012 by applying an appropriate voltage across the emitter cone 3012 and gate electrode 3014.
In another example of the construction of an FE-type element, the stacked structure of the kind shown in FIG. 18 is not used. Rather, the emitter and gate electrode are arranged on the substrate in a state substantially parallel to the plane of the substrate.
A known example of the MIM type is described by C.A. Mead, "Operation of tunnel emission devices", J. Appl. Phys., 32, 646 (1961). FIG. 19 is a sectional view illustrating a typical example of the construction of the MIM-type element. The element includes a substrate 3020, a lower electrode 3021 consisting of a metal, a thin insulating layer 3022 having a thickness on the order of 100 .ANG., and an upper electrode 3023 consisting of a metal and having a thickness on the order of 80.about.300 .ANG.. The element is caused to produce a field emission from the surface of the upper electrode 2023 by applying an appropriate voltage across the upper electrode 3023 and lower electrode 3021.
Since the above-mentioned cold cathode element makes it possible to obtain an electron emission element at a lower temperature in comparison with a hot cathode element, a heater for applying heat is unnecessary. Accordingly, the structure is simpler than that of the hot cathode element and it is possible to fabricate elements that are more slender. Further, even though a large number of elements are arranged on a substrate at a high density, problems such as fusing of the substrate do not readily arise. In addition, the cold cathode element differs from the hot cathode element in that the latter has a slow response speed because it is operated by heat produced by a heater. Thus, an advantage of the cold cathode element is a quicker response speed.
For these reasons, extensive research into applications for cold cathode elements is being carried out.
By way of example, among the various cold cathode elements, the surface-conduction electron emission element is particularly simple in structure and easy to manufacture and therefore is advantageous in that a large number of elements can be formed over a large area. Accordingly, research has been directed to a method of arraying and driving a large number of elements, as disclosed in Japanese Patent Application Laid-Open No. 64-31332, filed by the applicant.
Further, applications of surface-conduction electron emission elements that have been researched are image forming devices such as image display devices and image recording devices, as well as charged beam sources, etc.
As for applications to image display devices, research has been conducted with regard to such devices using, in combination, surface-conduction type electron emission elements and phosphors which emit light in response to irradiation with an electron beam, as disclosed, for example, in the specifications of U.S. Pat. No. 5,066,833 and Japanese Patent Application Laid-Open (KOKAI) Nos. 2-257551 and 4-28137 filed by the present applicant. The image display device using the combination of the surface-conduction type electron emission elements and phosphors is expected to have characteristics superior to those of the conventional image display device of other types. For example, in comparison with a liquid-crystal display device that has become so popular in recent years, the above-mentioned image display device emits its own light and therefore does not require back-lighting. It also has a wider viewing angle.
A method of driving a number of FE-type elements in a row is disclosed, for example, in the specification of U.S. Pat. No. 4,904,895 filed by the present applicant. A planar-type display apparatus reported by Meyer et al., for example, is known as an example of an application of an FE-type element to an image display apparatus. [R. Meyer: "Recent Development on Microtips Display at LETI", Tech. Digest of 4th Int. Vacuum Microelectronics Conf., Nagahama, pp. 6.about.9, (1991).]
An example in which a number of MIM-type elements are arrayed in a row and applied to an image display device is disclosed in the specification of Japanese Patent Application Laid-Open Nos. 3-55738 filed by the present applicant.
Among the available image forming apparatus that use electron emission elements of the kind described above, a flat panel display apparatus, which is very slender in the depth direction, is advantageous in that it occupies little space and is light in weight. For these reasons, such a display apparatus has become the focus of attention as an alternative to a display apparatus using a cathode-ray tube.
FIG. 20 is a perspective showing an example of the display panel portion of a flat-type image display apparatus. Part of the panel has been broken away to reveal the interior structure of the apparatus.
As shown in FIG. 20, the apparatus includes a rear plate 3115, a side wall 3116 and a face plate 3117. The rear plate 3115, side wall 3116 and face plate 3117 form a hermetic envelope for maintaining a vacuum within the display panel.
The substrate 3111 is fixed to the rear plate 3115 and N.times.M cold cathode elements 3112 are formed on the substrate. (N, M are positive integers having a value of two or greater, with the number being set appropriately in conformity with the number of display pixels intended.) The M.times.N cold cathode elements 3112 are wired by M-number of row-direction wiring patterns 3113 and N-number of column-direction wiring patterns 3114, as shown in FIG. 20. The portion constituted by the substrate 3111, cold cathode elements 3112, row-direction wiring patterns 3113 and column-direction wiring patterns 3114 is referred to as a "multiple electron beam source". Further, an insulating layer (not shown) is formed between the wiring patterns at least at the portions where the row-direction wiring patterns 3113 and column-direction wiring patterns 3114 intersect. This is to maintain the electrical insulation between the wiring patterns.
A phosphor film 3118 comprising phosphors is formed on the underside of the face plate 3117. Portions of the phosphor film 3118 are coated with individual phosphors (not shown) of the three primary colors red (R), green (G) and blue (B) Further, a black body (not shown) is provided between the individual color phosphors constituting the phosphor film 3118. A metal back 3119 comprising aluminum or the like is provided on the side of the phosphor film 3118 facing the rear plate 3115.
Electrical connection terminals Dx1.about.Dxm, Dy1.about.Dyn and Hv having an air-tight structure are provided to electrically connect the display panel to an electric circuit, which is not shown. The terminals Dx1.about.Dxm are electrically connected to the row-direction wiring patterns 3113 of the multiple electron beam source, the terminals Dy1.about.Dyn are electrically connected to the column-direction wiring patterns 3114 of the multiple electron beam source, and the terminal Hv is electrically connected to the metal back 3119.
The interior of the hermetic envelope is maintained at a vacuum on the order of 1.times.10.sup.-6 torr. An increase in the display area of the image display apparatus gives rise to the need for means for preventing deformation or breakage of the rear plate 3115 and face plate 3117 caused by a difference in air pressure between the interior and exterior of the hermetic envelope. A method that relies upon thickening of the rear plate 3115 and face plate 3116 not only increases the weight of the image display apparatus but also causes image deformation or parallax when the image is viewed from an oblique angle. By contrast, in FIG. 20, structural supports (referred to as "spacers" or "ribs") 3120 each comprising a comparatively thin glass plate for withstanding atmospheric pressure are provided. In this manner a gap usually on the order of less than one millimeter to several millimeters is maintained between the substrate 3111 on which the multiple electron beam source has been formed and the face plate 3166 on which the phosphor film 3118 has been formed, and the interior of the hermetic envelope is kept at a high vacuum.
When voltage is applied to each of the cold cathode elements 3112 through the external terminals Dx1.about.Dxm, Dx1.about.Dyn of the envelope in the image display apparatus using the above-described display panel, each of the cold cathode elements 3112 emits electrons. At the same time, a high voltage on the order of several hundred volts to several kilovolts is applied to the metal back 3119 through the external terminal Hv of the envelope, whereby the emitted electrons are accelerated and bombard the inner surface of the face plate 3117. As a result, the phosphors of the various colors constituting the phosphor film 3118 are excited into emitting light to display an image.
The display panel of the above-described image display apparatus has a number of problems, set forth below.
First, there is the possibility that the spacer 3120 will develop a charge owing to the fact that some of the electrons emitted from the vicinity of the spacer 3120 strike the spacer or the fact that ions produced by the ionizing effect of the emitted electrons attach themselves to the spacer. The paths of the electrons emitted by the cold cathode elements 3112 are caused to bend by the charge on the spacer and the electrons therefore arrive at locations on the phosphors that are different from the normal positions. As a consequence, the image in the vicinity of the spacer is displayed is distorted fashion.
Second, since a high voltage greater than several hundred volts (namely a strong electric field greater than 1 kV/mm) is impressed across the multiple electron beam source and the face plate 3117 in order to accelerate the electrons emitted by the cold cathode elements 3112, there is the danger that a surface discharge will occur on the surface of the spacer 3120. In a case where the spacer develops a charge in the manner described above, especially there is the possibility that a discharge will be induced.
In order to solve these problems, it has been proposed to eliminate the charge by arranging it so that a very small current flows into the spacer. To this end, a high-resistance film is formed on the surface of an insulating spacer, whereby a very small current flows on the surface of the spacer. The film used for preventing the spacer from being charged is a thin film of tin oxide, a mixed-crystal thin film of tin oxide and indium oxide, or an island-like metal film. Further, in order to enhance the function of the film used for preventing the spacer from being charged, it has been contemplated to dispose a conductive film on the surface of the spacer 3120 that contacts the substrate 3111 or the phosphor film 3118 and in the vicinity thereof. It is expected that this will assure an electrical connection between the film used for preventing the spacer from being charged and the substrate 3111 and between the film used for preventing the spacer from being charged and the phosphor film 3118.
However, if the conductive film has a projecting or angular shape, concentration of an electric field will occur when a high voltage is impressed across the substrate 3111 and face plate 3117. This may become a cause of discharge. As a result, a problem which arises is that the cold cathode elements 3112 are caused to deteriorate, making it difficult to form an image. If the voltage applied across the substrate 3111 and face plate 3117 is lowered in order to suppress such discharge, sufficient brightness can no longer be obtained.